In chemistry, epimers are diastereomers that differ in configuration of only one stereogenic center. Diastereomers are a class of stereoisomers that are non-superposable, non-mirror images of one another.

In chemical nomenclature, one of the epimeric pairs is given the prefix epi- for example in quinine and epi-quinine. When the pairs are enantiomers, the prefix becomes ent-.

The sugars α-glucose and β-glucose are epimers. In α-glucose, the -OH group on the first (anomeric) carbon is in the direction opposite the methylene group on carbon C-6 (in the axial position). In β-glucose, the -OH group is oriented in the same direction as the methylene group (in the equatorial position). These two molecules are both epimers and anomers.

β-D-glucopyranose and β-D-mannopyranose are epimers because they differ only in the stereochemistry at the C-2 position. The hydroxyl group in β-D-glucopyranose is equatorial (in the "plane" of the ring) while in β-D-mannopyranose the C-2 hydroxyl group is axial (up from the "plane" of the ring). These two molecules are epimers but not anomers.